Deep Purple singer Ian Gillan and Black Sabbath guitarist Tony Iommi are about to release a somewhat confusing collection that includes lesser-known material from both artists’ careers, as well as some of their collaborations. Jokingly titled WhoCares (after a side project that both artists were involved in), it’s likely that heavy metal and hard rock fans will care a lot.

The collection highlights some of the intersections between Purple and Sabbath, two of the most influential bands in those genres. Among the tracks on the album:

Black Sabbath – “Zero The Hero”: One of the songs from their 1983 album Born Again, which featured Ian Gillan on vocals. Gillan left after the tour supporting the album to rejoin Deep Purple.

Ian Gillan featuring Tony Iommi, Ian Paice and Roger Glover – “Trashed”: An updated version of the Born Again song with Gillan’s Deep Purple mates Paice and Glover. This version was from Gillan’s 2006 album Gillan’s Inn, which saw him revisiting songs from his entire career.

Tony Iommi and Glenn Hughes – “Slip Away” from the Iommi/Hughes 2005 album Fused. Weirdly, former Deep Purple bass player replaced Gillan as Sabbath’s singer in 1986 for the Seventh Star album (originally intended to be Iommi’s solo debut, it became a Black Sabbath album at the insistence of the record label). Hughes didn’t last long in Sabbath, and was fired during the tour to support the album. Nearly two decades later, the two got back together to collaborate on the Fused album.

Deep Purple featuring Ronnie James Dio live with the Royal Philharmonic Orchestra – “Smoke On The Water”: OK, follow this: Ronnie James Dio used to sing for a band called Elf who opened for Deep Purple. When Purple’s guitarist Ritchie Blackmore quit to form Rainbow, he drafted the members of Elf, including Dio, to be a part of his new group. Dio later left Rainbow to replace Ozzy Osbourne in Black Sabbath. When Dio left Sabbath, Gillan replaced him. This performance from a 1999 concert, brings lots of old friends back together.

Ian Gillan & Roger Glover – “Can’t Believe You Wanna Leave Me”: Deep Purple’s singer and bass player did the Accidentally On Purpose album during a break between DP albums in 1988; this track featured Dr. John on piano.

Two tracks by WhoCares, which featured Iommi and Gillan with former Deep Purple keyboardist Jon Lord (who left the band in 2002 and passed away earlier this year), former Metallica bassist Jason Newstead and Iron Maiden drummer Nicko McBrain. The WhoCares tracks were done to raise funds to finance the rebuilding of a music school in Armenia.